Graphite oxide provided with an oxygen-containing functional group is prepared by oxidizing graphite, which has a layered structure in which carbon atoms are bonded to each other through sp2 bonding and arranged in planes. Many studies have been conducted on graphite oxide for its unique structure and properties. Graphite oxide is expected to be used as catalysts, electrode active materials for cells and capacitors, thermoelectric conversion materials, conductive materials, light emitting materials, lubricant additives, additives for polymers, permeable membrane materials, antimicrobial materials, water repellent materials, and adsorption materials. For example, graphite oxide is desirably dispersed in oil to be used as an additive for machine lubricating oil.
Such graphite oxide may be derivatized to obtain additional desired functions. Graphite oxide is, for example, desirably dispersed in oil to be used as a lubricant additive or used in the form of a composite with resin such as a polymer. However, hydrophilic graphite oxide as it is cannot be sufficiently dispersed in a non- or low-polar dispersion medium such as oil or resin (hydrophobic dispersion medium). Therefore, graphite oxide is treated to be made dispersible. Specifically, a hydrophilic oxygen-containing functional group is reduced by heating or chemical reaction, or graphite oxide is modified with a hydrophobic substituent. There are some reports on reduction reaction of graphite oxide or modification (introduction) reaction of a substituent (see Patent Documents 1 to 3, Non-Patent Documents 1 to 8). In particular, in Non-Patent Document 3, examination is conducted for enhancement of the dispersibility of graphite oxide in a non-polar dispersion medium, but purification and drying are performed as pretreatment, reduction treatment is performed to aggressively remove reactive oxygen, and a strongly basic reagent is used to perform treatment before reaction.
As a method for producing graphite oxide, a method in which graphite oxide is synthesized by reacting graphite with a strong oxidant in an acid solvent and the resulting graphite oxide is purified from the solution is generally used, and the Hummers method is known in which sulfuric acid and potassium permanganate are used as oxidants (see Non-Patent Document 9, Patent Documents 4 to 6). In addition to this method, the Brodie method in which nitric acid and potassium chlorate are used and the Staudenmaier method in which sulfuric acid, nitric acid, and potassium chlorate are used as oxidants are known. After oxidation of graphite, graphite oxide is generally separated and purified from the reaction liquid containing graphite oxide by centrifuging or filtering the reaction liquid. Filtering under gas pressure has been reported as a method for efficiently separating and purifying graphite oxide (see Non-Patent Documents 10 and 11).